Modern Portfolio Theory (MPT), developed in the 1950s by Nobel Prize Winner Markowitz, illustrates that a portfolio of assets has different risk characteristics than the simple sum of the risks of the individual assets. Though risk can be defined in many ways, the example of MPT serves to identify a general matter of concern to investors. Under MPT, risk is defined as the standard deviation (or, alternatively the variance) on the period-to-period returns of an asset. For example, a stock with an expected return of 12% per year, and volatility expressed as a standard deviation of 15%, is expected to have returns within one standard deviation (about 68% of the time) between 12+15=27% and 12−15=−3% in any year. When numerous assets are combined in a portfolio, these standard deviations cannot simply be added up. Typically, the assets are somewhat uncorrelated, so as group, the volatilities of individual assets counteract each other, in a manner known as risk diversification. Although the long-term, average expected returns for each asset do sum up for the portfolio, the volatility of these returns will result in actual returns of some assets to increase above the expected rate, and to decrease below the expected rate for other assets, for any period of time. The combined effect is a portfolio return that has lower volatility than that of the individual returns, while the expected year-to-year return (i.e., rate of growth) remains the same.
The general principles of portfolio diversification assist in identifying a mix of assets that generate the highest expected returns at the lowest portfolio risk. Portfolio risk can be minimized by selecting assets whose volatilities are less correlated. Portfolio returns are defined by the individual expected returns of the assets. Using commercially available optimization software, one can allocate the mix of certain liquid assets (e.g., stocks, bonds, etc.) in a portfolio to achieve optimal performance, where “optimal” is typically defined as some combination of objectives for the volatility and returns of the portfolio.
Privately owned residential real estate comprises roughly $20 trillion of US wealth. By and large, this investment is not managed from a portfolio allocation standpoint, and is not accounted for in private and institutional portfolios using explicit quantitative methods of portfolio diversification. In this context, “explicit” and “quantitative” imply that asset allocations in a portfolio are based on calculations using time series histories of value and rent income data for real estate properties.